Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion battery) is charging, Na+ (or L+) ions de-intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today; however lithium is not a cheap metal to source and is considered too expensive for use in large scale applications. By contrast sodium-ion battery technology is still in its relative infancy but is seen as advantageous; sodium is much more abundant than lithium and some researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless a lot of work has yet to be done before sodium-ion batteries are a commercial reality.
Metal oxides with the general formula AxMO2 (where A represents one or more alkali metal ions and M represents one or more metal ions at least one of which has several oxidation states, for example a transition metal) are known to crystallise in a number of different layered structures. This is described in detail by C. Delmas et al in “Structural Classification and Properties of the Layered Oxides”, Physica 99B (1980) 81-85. In summary, the structures are all made up of MO6 edge sharing octahedra which form (MO2)n sheets. These sheets are stacked one on top of the other and are separated by the alkali metal atoms and the exact position of the alkali metal will dictate whether the overall structure of the metal oxide is to be described as octahedral (O), tetrahedral (T) or prismatic (P). In a lattice made up of hexagonal sheets, there are three possible positions for the oxygen atoms, conventionally named A, B and C. It is the order in which these sheets are packed together that leads to the O, T and P environments. The number 2 or 3 is also used to describe the number of alkali metal layers in the repeat unit perpendicular to the layering. For example, when the layers are packed in the order ABCABC, an O3 structure is obtained. This translates to 3 alkali metal layers in the repeat unit and each alkali metal being in an octahedral environment. Such materials are characterised by the alkali metal ions being in octahedral orientation and typical compounds of this structure are AxMO2 (x≤1). The order ABAB with the alkali metal ions in tetrahedral orientation will yield a T1 structure which is typified by A2MO2 compounds. Packing the sheets in ABBA order gives a P2 structure in which one half of the prism shares edges with MO6 octahedra and the other half shares faces and typical compounds are A=0.7MO2. And finally, packing in ABBCCA order results in a P3 structure type in which all prisms share one face with one MO6 octahedron and three edges with three MO6 octahedra of the next sheet. A=0.5MO2 compounds are found to adopt the P3 structure. It will be noted that the amount of alkali metal present in the AxMO2 material has a direct bearing on the overall structure of the metal oxide.
Further, Y. J. Shin et al. report in Solid State Ionics 132 (2000) 131-141, the preparation and structural properties of layer-type oxides NaxNix/2Ti1−x/2O2, in which x is in the range 0.6≤x≤1.0. In particular, these workers disclose that rhombohedral (type O) is observed when 0.72<x≤1.0 and hexagonal lattice (type P) is observed when 0.6≤x≤0.72, and that both structure types O and P are present as a mixture when the product is made in a solid state process at around 1223 K (approximately 950° C.).
Over the last ten years, numerous workers have investigated the electrochemical properties of single phase metal oxides with either P2 or O3 structures. For example C. Delmas et al report the phase transformations and electrochemical behaviour of P2-NaxCoO2, see for example J. Solid State Chem., 2004, 177, 2790-2802 and Inorg. Chem., 2009, 48, 9671-9683. Further, Delmas et al have reported that although layered O3 type materials NaxVO2, NaxCrO2, NaxMnO2 and NaxFeO2 are able to host Na-ions upon charge and discharge and have excellent specific capacity performance, they nevertheless suffer significant capacity fading. Lu and Dahn, J. Electrochem. Soc., 2001, 148, A710-715, demonstrate that the P2-layered oxide Na2/3[Ni1/3Mn2/3]O2 can reversibly exchange Na-ions in sodium half cells however, these oxide compounds are expected to show poor cycling ability, especially between 2.3-4.5 V at C/100.
More recently, Kim et al Adv. Energy Mater., 2011, 1, 333-336 report that the presence of lithium in single phase P2 lithium substituted compounds such as Na1.0Li0.2Ni0.25Mn0.75O2, provides some improvement in the structural stability during cycling, but the reversible capacity of these compounds is still too low due to the limited amount (25%) of redox active divalent Ni. An attempt to increase the capacity to be closer to the theoretical value of 180 mAhg−1 is reported by Kim et al in an abstract of their presentation to be given at The 17th International Meeting on Lithium Batteries Jun. 10-14, 2014 Como, Italy, and involves using Na1−xLixNi0.5Mn0.5O2(Na/Li=1.0). During the course of this work, Kim et al note the presence of an intergrowth of P2 and O3 layered phases in this material which they hypothesize, stabilises the crystal structure and leads to improved reversible capacity. The best capacity results are reported for the x=0.3 compound, which also corresponds as being the compound with the highest percentage of P2. The x=0 material which is O3 stacked, is the lowest performer. In another recent paper by Y. Shirley Meng and D. H. Lee, Phys. Chem. Chem. Phys., 2013, 15, 3304, P2-Na2/3[Ni1/3Mn2/3]O2 is reported to exhibit excellent cycling and a high rate capability, however these results are only achieved when the material is charged below 4.22V; above 4.22V, the charge capacity in not maintained during cycling due to the phase transformation from P2 to O2.
In conclusion, the metal oxides studied discussed above are hampered either by low specific charge capacity or poor cycling stability especially across a wide range of charge voltages, and as a consequence the commercial application of these compounds in Na ion cells is limited.
The current workers have developed novel electrodes comprising particular doped-nickelate-containing compositions that are capable of delivering excellent specific capacity performance, in conjunction with little or no fading on cycling. Moreover, the doped-nickelate-containing compositions used in the electrodes of the present invention have been found to achieve these excellent results under voltage conditions that would typically result in their phase transformation from P2 to O2; this is a significant improvement over compounds used in the electrodes described in the prior art. Thus the present invention may be used to provide electrodes which are able to be recharged multiple times without significant loss in charge capacity. Advantageously, these electrodes may be used in batteries, especially rechargeable batteries, electrochemical devices and electrochromic devices.
The present invention therefore provides an electrode comprising doped nickelate-containing compositions comprising a first component type comprising one or more components with an O3 structure of the general formula:AaM1VM2WM3XM4yM5ZO2                 wherein        A comprises one or more alkali metals selected from sodium, lithium and potassium;        M1 is nickel in oxidation state 2+,        M2 comprises one or more metals in oxidation state 4+,        M3 comprises one or more metals in oxidation state 2+,        M4 comprises one or more metals in oxidation state 4+, and        M5 comprises one or more metals in oxidation state 3+        wherein        0.85≤a≤1, preferably 0.90≤a≤1 and further preferably 0.95≤a≤1;        0<v<0.5, preferably 0<v≤0.45 and ideally 0<v≤0.333;        at least one of w and y is >0;        x≥0, preferably x>0;        z≥0;        and wherein a, v, w, x, y and z are chosen to maintain electroneutrality;        together with one or more component types selected from        a second component-type comprising one or more components with a P2 structure of the general formula:A′a′M1′V′M2′W′M3′X′M4′y′M5′Z′O2         wherein        A′ comprises one or more alkali metals selected from sodium, lithium and potassium;        M1′ is nickel in oxidation state 2+,        M2′ comprises one or more metals in oxidation state 4+,        M3′ comprises one or more metals in oxidation state 2+,        M4′ comprises one or more metals in oxidation state 4+, and        M5′ comprises one or more metals in oxidation state 3+        wherein        0.4≤a′<1, preferably 0.5≤a′<0.85, further preferably 0.6≤a′≤0.7;        0<v′<0.5, preferably 0<v′≤0.45 and ideally 0<v′≤0.333;        at least one of w′ and y′ is >0;        x′≥0, preferably x′>0;        z′≥0;        and wherein a′, v′, w′, x′, y′ and z′ are chosen to maintain electroneutrality;        and a third component-type comprising one or more components with a P3 structure of the general formula:A″a″M1″V″M2″W″M3″X″M4″y″M5″Z″O2         wherein        A″ comprises one or more alkali metals selected from sodium, lithium and potassium;        M1″ is nickel in oxidation state 2+,        M2″ comprises one or more metals in oxidation state 4+,        M3″ comprises one or more metals in oxidation state 2+,        M4″ comprises one or more metals in oxidation state 4+, and        M5″ comprises one or more metals in oxidation state 3+        wherein        0.4≤a″<1, preferably 0.5≤a″<0.85, further preferably 0.6≤a″≤0.7;        0<v″<0.5, preferably 0<v″≤0.45 and ideally 0<v″≤0.333;        at least one of w″ and y″ is >0;        x″≥0, preferably x″>0;        z″≥0;        and wherein a″, v″, w″, x″, y″ and z″ are chosen to maintain electroneutrality.        
Preferably the alkali metal A and/or A′ and/or A″ is selected from either sodium or a mixed alkali metal in which sodium is the major constituent.
Preferred electrodes comprise doped nickelate-containing compositions comprising:                a first component-type comprising one or more components with an O3 structure and of the general formula:AaM1vM2wM3xM4yM5zO2         wherein        A comprises one or more alkali metals selected from sodium, lithium and potassium;        M1 is nickel in oxidation state 2+,        M2 comprises one or more metals in oxidation state 4+,        M3 comprises one or more metals in oxidation state 2+,        M4 comprises one or more metals in oxidation state 4+, and        M5 comprises one or more metals in oxidation state 3+        wherein        0.95≤a≤1;        0.3≤v≤0.333;        at least one of w and y is >0;        x>0;        z≥0;        and wherein a, v, w, x, y and z are chosen to maintain electroneutrality;        and        a second component-type comprising one or more components with a P2 structure of the general formula:A′a′M1′V′M2′W′M3′X′M4′y′M5′Z′O2         wherein        A′ comprises one or more alkali metals selected from sodium, lithium and potassium;        M1′ is nickel in oxidation state 2+,        M2′ comprises one or more metals in oxidation state 4+,        M3′ comprises one or more metals in oxidation state 2+,        M4′ comprises one or more metals in oxidation state 4+, and        M5′ comprises one or more metals in oxidation state 3+        wherein        0.6<a′<0.85;        0.25<v′≤0.333;        at least one of w′ and y′ is >0;        x′>0;        z≥0;        and wherein a′, v′, w′, x′, y′ and z′ are chosen to maintain electroneutrality.        
Preferred first components with an O3 structure include:Na1−σNi(1−σ)/3Mn1−σ)/3Mg(1/6)−(1/6)σTi(1/6)+(5/6)σO2,                where 0≤σ≤0.15;        preferably 0.001≤σ≤0.05,Na0.95Ni0.3167Mn0.3167Mg0.1583Ti0.2083O2 andNaNi0.33Mn0.33Mg0.167Ti0.167O2.        
Preferred second components with a P2 structure include:Na0.67Ni0.33Mn0.67O2,Na0.67Ni0.3Mn0.6Mg0.033Ti0.067O2,Na0.67Ni0.267Mn0.533Mg0.0067Ti0.133O2,Na0.67Ni0.25Mg0.083Mn0.667O2,Na0.67Ni0.283Mn0.567Mg0.05Ti0.1O2,Na0.67Ni0.25Mn0.667Mg0.083O2.
Preferred third components with a P3 structure include:Na0.667Ni0.25Mn0.65Mg0.0835Ti0.0165O2,Na0.6Ni0.28Mn0.6530Mg0.02Ti0.047O2,Na0.7Ni0.32Mn0.594Mg0.03Ti0.056O2,Na0.667Ni0.25Mn0.5Mg0.0835Ti0.1165O2.
Metals M2 and M4 may be the same or different metal(s) in oxidation state 4+. Moreover M2 and M4 are interchangeable with each other. When M2=M4, then the first component-type comprising one or more components with an O3 structure, may be written either as:AaM1VM2WM3XM4YM5ZO2,orAaM1VM2W+YM3XM5ZO2,orAaM1VM3XM4Y+WM5ZO2,and all of these forms of the equation are to be regarded as equivalent.
The same is true for M2′ and M4′ in the second component-type comprising one or more components with a P2 structure, and also for M2″ and M4″ in the third component-type comprising one or more components with a P3 structure.
Preferably at least one of the components in one of the first, second and third component-types of the doped nickelate-containing compositions used in the electrodes of the present invention comprise sodium alone as the chosen alkali metal.
Also in further preferred electrodes of the present invention, each of M2, M2′ and M2″ in the doped nickelate-containing compositions, comprise one or more metals in oxidation state 4+ selected from manganese, titanium and zirconium; each of M3, M3′ and M3″ comprise one or more metals in oxidation state 2+ selected from magnesium, calcium, copper, zinc and cobalt; each of M4, M4′ and M4″ comprise one or more metals in oxidation state 4+ selected from manganese, titanium and zirconium; and each of M5, M5 and M5″ comprise one or more metals in oxidation state 3+ selected from aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium.
There is a continuum of possible ratios between the first component-type (O3): second component-type (P2): third component-type (P3) in doped nickelate-containing compositions used in the electrodes of the present invention and this will vary depending on the choice of first, second, and third component-types. In practice, the ratio which is chosen is the one found by experiment to provide the required excellent specific capacity and cycling performance. Examples of suitable ratios include:    1(O3):1(P2):0(P3),    1(O3):3(P2):0(P3),    3(O3): 1(P2):0(P3),    1(O3):1(P2):1(P3).
The doped nickelate-containing compositions used in the electrodes of the present invention may be prepared by admixing a first component type comprising one or more components with an O3 structure of the general formula:AaM1VM2WM3XM4yM5ZO2                 wherein A comprises one or more alkali metals selected from sodium, lithium and potassium;        M1 is nickel in oxidation state 2+,        M2 comprises one or more metals in oxidation state 4+,        M3 comprises one or more metals in oxidation state 2+,        M4 comprises one or more metals in oxidation state 4+, and        M5 comprises one or more metals in oxidation state 3+,        wherein        0.85≤a≤1, preferably 0.90≤a≤1 and further preferably 0.95<a≤1;        0<v<0.5, further preferably 0<v≤0.45 and ideally 0<v≤0.333;        at least one of w and y is >0;        x≥0, preferably x>0;        z≥0;        and wherein a, v, w, x, y and z are chosen to maintain electroneutrality;        together with one or more component types selected from:        a second component-type comprising one or more components with a P2 structure of the general formula:A′a′M1′V′M2′W′M3′X′M4′y′M5′Z′O2         wherein        A′ comprises one or more alkali metals selected from sodium, lithium and potassium;        M1′ is nickel in oxidation state 2+,        M2′ comprises one or more metals in oxidation state 4+,        M3′ comprises one or more metals in oxidation state 2+,        M4′ comprises one or more metals in oxidation state 4+, and        M5′ comprises one or more metals in oxidation state 3+        wherein        0.4≤a′<1, preferably 0.5≤a′<0.85, further preferably 0.6≤a′≤0.7;        0<v′<0.5, preferably 0<v′≤0.45 and ideally 0<v′≤0.333;        at least one of w′ and y′ is >0;        x′≥0, preferably x′>0;        z′≥0;        and wherein a′, v′, w′, x′, y′ and z′ are chosen to maintain electroneutrality;        and a third component-type comprising one or more components with a P3 structure of the general formula:A″a″M1″V″M2″W″M3″X″M4″y″M5″Z″O2         wherein        A″ comprises one or more alkali metal selected from sodium, lithium and potassium;        M1″ is nickel in oxidation state 2+,        M2″ comprises one or more metals in oxidation state 4+,        M3″ comprises one or more metals in oxidation state 2+,        M4″ comprises one or more metals in oxidation state 4+, and        M5″ comprises one or more metals in oxidation state 3+        wherein        0.4≤a″<1, preferably 0.5≤a″<0.85, further preferably 0.6≤a″≤0.7;        0<v″<0.5, preferably 0<v″≤0.45 and ideally 0<v″≤0.333;        at least one of w″ and y″ is >0;        x″≥0, preferably x″>0;        z″≥0;        and wherein a″, v″, w″, x″, y″ and z″ are chosen to maintain electroneutrality.        
The above method produces doped nickelate-containing compositions which comprise a physical admixture of separate components (which in this case are separate compounds) which make up the first, second and third component-types. The separate compounds are prepared using any convenient method, for example a solid state method at 500-1200′C. Suitable methods are also described in PCT/GB2013/051821, PCT/GB2013/051822, PCT/2013/051808, PCT/GB2013/051824, and PCT/GB2013/052620.
The components of the first, second and third component-types may be admixed using any known method. Preferably, however, admixing is performed by solid state mixing, for example using a pestle and mortar, a micronizer or a mixer mill It is found useful to use a dispersant (such as a low boiling material, for example acetone) assist the mixing process, although this dispersant should be at least substantially removed prior to the synthesis, i.e. prior to the heating step. It is particularly advantageous to ensure that the components of the first component type and the components of one or both of the second and third component-types are intimately mixed together. It is also possible to mix the components of the first component-type with the components of one or both of the second and third component-types during the process for making electrode slurries.
In an alternative method, the doped nickelate-containing compositions used in the electrodes of the present invention are directly synthesized by reacting together precursor materials of the components of the first component-type, with the precursor materials of one or both of the second and third components of the second and third component-types, respectively, to yield a single compound in which components of the first, second and third component-types are present. Such a single compound, may comprise discrete regions within its structure of the first component-type comprising one or more components with an O3 structure as defined above, together with one or both of a second component-type comprising one or more components with a P2 structure as defined above and a third component-type comprising one or more components with a P3 structure as defined above. It is also possible that one or more of the first, second and third component-types are pre-made before being mixed with the precursor materials for the remaining first, second or third component-types, as required for the desired composition.
Thus, in a second aspect, the invention provides a process for making the doped nickelate-containing compositions comprising the chemical and/or physical mixing of a first component-type comprising either one or more components with an O3 structure of the general formula:AaM1VM2WM3XM4yM5ZO2                 wherein        A comprises one or more alkali metals selected from sodium, lithium and potassium;        M1 is nickel in oxidation state 2+,        M2 comprises one or more metals in oxidation state 4+,        M3 comprises one or more metals in oxidation state 2+,        M4 comprises one or more metals in oxidation state 4+, and        M5 comprises one or more metals in oxidation state 3+        Wherein        0.85≤a≤1, preferably 0.90≤a≤1 and further preferably 0.95<a≤1;        0<v<0.5, further preferably 0<v≤0.45 and ideally 0<v≤0.333;        at least one of w and y is >0;        x≥0, preferably x>0;        z≥0′        wherein a, v, w, x, y and z are chosen to maintain electroneutrality;        and/or the precursor materials for preparing the one more components with an O3 structure;        together with one or more component-types selected from:        a second component-type comprising either one or more components with a P2 structure of the general formula:A′a′M1′V′M2′W′M3′X′M4′y′M5′Z′O2         wherein        A′ comprises one or more alkali metals selected from sodium, lithium and potassium;        M1′ is nickel in oxidation state 2+,        M2′ comprises one or more metals in oxidation state 4+,        M3′ comprises one or more metals in oxidation state 2+,        M4′ comprises one or more metals in oxidation state 4+, and        M5′ comprises one or more metals in oxidation state 3+        wherein        0.4≤a′<1, preferably 0.5≤a′<0.85, further preferably 0.6≤a′≤0.7;        0<v′<0.5, preferably 0<v′≤0.45 and ideally 0<v′≤0.333;        at least one of w′ and y′ is >0;        x′≥0, preferably x′>0;        z′≥0;        wherein a′, v′, w′, x′, y′ and z′ are chosen to maintain electroneutrality;        and/or the precursor materials for preparing the one more components with a P2 structure;        and a third component-type comprising either one or more components with a P3 structure of the general formula:A″a″M1″V″M2″W″M3″X″M4″y″M5″Z″O2         wherein        A″ comprises one or more alkali metal selected from sodium, lithium and potassium;        M1″ is nickel in oxidation state 2+,        M2″ comprises one or more metals in oxidation state 4+,        M3″ comprises one or more metals in oxidation state 2+,        M4″ comprises one or more metals in oxidation state 4+, and        M5″ comprises one or more metals in oxidation state 3+        wherein 0.4≤a″<1, preferably 0.5≤a″<0.85, further preferably 0.6≤a″≤0.7;        0<v″<0.5, preferably 0<v″≤0.45 and ideally 0<v′≤0.333;        at least one of w″ and y″ is >0;        x″≥0, preferably x″>0;        z″≥0;        wherein a″, v″, w″, x″, y″ and z″ are chosen to maintain electroneutrality;        and/or the precursor materials for preparing the one more components with a P3 structure.        
The doped nickelate-containing compositions used in the electrodes of the present invention are conveniently described by a formula that uses a weighted average of the first component-type, together with one or more of the second and third component-types. For example a doped nickelate-containing composition with a first component-type comprising an O3 compound such as O3-NaNi0.33Mn0.33Mg0.167Ti0.167O2, and a second component-type comprising a P2 compound such as P2-Na0.67Ni0.300Mn0.600Mg0.033Ti0.067O2 (where O3:P2 is in the ratio 1:1) can be described by the following weighted average formula: Na0.833Ni0.317Mn0.467Mg0.100Ti0.117O2.
It is worth noting that when the doped nickelate-containing compositions are made by chemical mixing, it is likely that the exact structure of each of the components of the first, second and third component-types will, in practice, be determined by whichever is the most thermodynamically stable structure for the O3, P2 and P3 phases, and this will be based on the ratio of the precursor materials used. Thus, in the above example, the O3 and P2 phases may be represented by Na1−εNi0.33±εMn0.33±εMg0.167±εTi0.167±εO2 and Na0.67±εNi0.300±εMn0.600±εMg0.033±εTi0.067±εO2 respectively, where ε refers to an unknown quantity.
In a third aspect, the present invention provides electrodes comprising doped nickelate-containing compositions comprising a first component-type comprising one or more components with an O3 structure together with one or more component types selected from a second component-type comprising one or more components with a P2 structure, and a third component-type comprising one or more components with an P3 structure, and with a weighted average formula represented by the general formula:A′″a′″M1′″V′″M2′″W′″M3′″X′″M4′″y′″M5′″Z′″O2                 wherein        A′″ comprises one or more alkali metals selected from sodium, lithium and potassium;        M1′″ is nickel in oxidation state 2+,        M2′″ comprises one or more metals in oxidation state 4+,        M3′″ comprises one or more metals in oxidation state 2+,        M4′″ comprises one or more metals in oxidation state 4+, and        M5′″ comprises one or more metals in oxidation state 3+        wherein        0.4≤a′″<1, preferably 0.5≤a′″≤0.95, further preferably 0.6≤a′″≤0.9 and ideally 0.7≤a′″≤0.9;        0<v′″<0.5, further preferably 0<v′″≤0.45, ideally 0<v′″≤0.333 and alternatively 0.2≤v′″≤0.333;        at least one of w′″ and y′″ is >0;        x′″≥0, preferably x′″>0;        z′″≥0;        and wherein a′″, v′″, w′″, x′″, y′″ and z′″ are chosen to maintain electroneutrality.        
A′″ is the same as defined above for A, A′ and A″; M1′″ is the same as defined above for M1, M1′ and M1″; M2′″ is the same as defined above for M2, M2′ and M2″; M3′″ is the same as defined above for M3, M3′ and M3″; M4′″ is the same as defined above for M4, M4′ and M4″; and M5′″ is the same as defined above for M5, M5′ and M5″.
Preferred doped nickelate-containing compositions used in the electrodes of the present invention are described by the following weighted average formulae:O3/P2-Na0.833Ni0.317Mn0.467Mg0.100Ti0.117O2,O3/P2-Na0.750Ni0.296Mn0.508Mg0.079Ti0.117O2,O3/P2-Na0.85Ni0.4Mn0.5Mg0.025Ti0.075O2,O3/P2-Na0.95Ni0.3167Mn0.3167Mg0.1583Ti0.2083O2,O3/P2-Na0.8Ni0.2667Mn0.2667Mg0.1333Ti0.3333O2,O3/P2-Na0.75Ni0.25Mn0.25Mg0.125Ti0.375O2, andO3/P2-Na0.7Ni0.2333Mn0.2333Mg0.1167Ti0.4167O2.
The electrodes of the present invention are suitable for use in many different applications including sodium ion and/or lithium ion and/or potassium ion cells which may be widely used for example in energy storage devices, such as batteries, rechargeable batteries, electrochemical devices and electrochromic devices. Preferably the electrodes of the present invention may be used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s).
Advantageously, the electrodes of the present invention are cathode electrodes.
In a fourth aspect, the present invention provides for the use of electrodes that comprise doped nickelate-containing compositions as described above, in energy storage devices, such as batteries, rechargeable batteries, electrochemical devices and electrochromic devices.
In fifth aspect, the present invention provides energy storage devices such as batteries, rechargeable batteries, electrochemical devices and electrochromic devices that comprise an electrode comprising doped nickel-containing compositions as described above.
In a sixth aspect, the present invention provides doped nickelate-containing compositions comprising one or more component types selected from:                a first component-type comprising one or more components with an O3 structure of the general formula:AaM1VM2WM3XM4yM5ZO2        wherein        A comprises one or more alkali metal selected from sodium, lithium and potassium;        M1 is nickel in oxidation state 2+,        M2 comprises one or more metals in oxidation state 4+,        M3 comprises one or more metals in oxidation state 2+,        M4 comprises one or more metals in oxidation state 4+, and        M5 comprises one or more metals in oxidation state 3+        wherein        0.85≤a≤1, preferably 0.90≤a≤1 and further preferably 0.95≤a≤1;        0<v<0.5, preferably 0<v≤0.333;        at least one of w and y is >0;        x≥0;        z≥0;        and wherein a, v, w, x, y and z are chosen to maintain electroneutrality;        a second component-type comprising one or more components with a P2 structure of the general formula:A′a′M1′V′M2′W′M3′X′M4′y′M5′Z′O2         wherein        A′ comprises one or more alkali metal selected from sodium, lithium and potassium;        M1′ is nickel in oxidation state 2+,        M2′ comprises one or more metals in oxidation state 4+,        M3′ comprises one or more metals in oxidation state 2+,        M4′ comprises one or more metals in oxidation state 4+, and        M5′ comprises one or more metals in oxidation state 3+        wherein        0.4≤a′<1, preferably 0.5≤a′<0.85; further preferably 0.6≤a′≤0.7;        0<v′<0.5, preferably 0<v′≤0.45 and ideally 0<v′≤0.333;        at least one of w′ and y′ is >0;        x′≥0, preferably x′>0;        z′≥0;        and wherein a′, v′, w′, x′, y′ and z′ are chosen to maintain electroneutrality;        and a third component-type comprising one or more components with a P3 structure of the general formula:A″a″M1″V″M2″W″M3″X″M4″y″M5″Z″O2         wherein        A″ comprises one or more alkali metals selected from sodium, lithium and potassium;        M1″ is nickel in oxidation state 2+,        M2″ comprises one or more metals in oxidation state 4+,        M3″ comprises one or more metals in oxidation state 2+,        M4″ comprises one or more metals in oxidation state 4+, and        M5″ comprises one or more metals in oxidation state 3+        wherein        0.4≤a″<1, preferably 0.5≤a″<0.85, further preferably 0.6≤a″≤0.7;        0<v″<0.5, preferably 0<v″≤0.45 and ideally 0<v″≤0.333;        At least one of w″ and y″ is >0;        x″≥0;        z″≥0;        and wherein a″, v″, w″, x″, y″ and z″ are chosen to maintain electroneutrality.        
In preferred doped nickelate-containing compositions of the present invention, at least one of x, x′ and x″ is >0
The doped nickelate-containing composition according to the sixth aspect of the present invention can be represented by a weighted average formula:A′″a′″M1′″V′″M2′″W′″M3′″X′″M4′″y′″M5′″Z′″O2                 wherein        A′″ comprises one or more alkali metals selected from sodium, lithium and potassium;        M1′″ is nickel in oxidation state 2+,        M2′″ comprises one or more metals in oxidation state 4+,        M3′″ comprises one or more metals in oxidation state 2+,        M4′″ comprises one or more metals in oxidation state 4+, and        M5′″ comprises one or more metals in oxidation state 3+        wherein        0.4≤a′″<1, preferably 0.5≤a′″≤0.95, further preferably 0.6≤a′″≤0.9 and ideally 0.7≤a′″≤0.9;        0<v′″<0.5, further preferably 0<v′″≤0.45, ideally 0<v′″≤0.333 and alternatively 0.2≤v′″≤0.333;        at least one of w′″ and y′″ is >0;        x′″≥0, preferably x′″>0;        z′″≥0;        and wherein a′″, v′″, w′″, x′″, y′″ and z′″ are chosen to maintain electroneutrality.        
All of A, A′, A″, A′″, M1, M1′, M1″, M1′″, M2, M2′, M2″, M2′″, M3, M3′, M3″, M3′″, M4, M4′, M4″, M4′″, M5, M5′, M5″, and M5′″ are as defined above in relation to the doped nickelate-containing compositions used in the electrode of the present invention. As will follow from the above description, the compositions of the sixth aspect of the present invention will be either i) a single compound comprising discrete areas containing one or more components with an O3 structure, together with discrete areas of components with one or both of P2 and P3 structures, or ii) it will be a physical mixture comprising one or more compounds with an O3 structure together with one or more compounds with a P2 and/or a P3 structure, or iii) it will be a mixture of i) and ii).
The doped nickelate-containing compositions according to the sixth aspect of the present invention may be used in energy storage devices such as batteries, rechargeable batteries, electrochemical devices and electrochromic devices. Use in an electrode in such energy storage devices is preferred.
The most preferred doped nickelate-containing compositions according to the sixth aspect of the present invention are selected from:O3/P2-Na0.833Ni0.317Mn0.467Mg0.100Ti0.117O2,O3/P2-Na0.750Ni0.296Mn0.508Mg0.079Ti0.117O2,O3/P2-Na0.85Ni0.4Mn0.5Mg0.025Ti0.075O2,O3/P2-Na0.95Ni0.3167Mn0.3167Mg0.1583Ti0.2083O2,O3/P2-Na0.8Ni0.2667Mn0.2667Mg0.1333Ti0.3333O2,O3/P2-Na0.75Ni0.25Mn0.25Mg0.125Ti0.375O2,O3/P2-Na0.7Ni0.2333Mn0.2333Mg0.1167Ti0.4167O2.
The doped nickelate-containing compositions according to the sixth aspect, may be prepared according to the procedures described above.
When making doped nickelate-containing compositions it is possible to convert sodium-ion derivatives into mixed lithium-ion/sodium-ion materials using an ion exchange process.
Typical ways to achieve Na to Li-ion exchange include:    1. Mixing the sodium-ion material with a lithium-containing material e.g. LiNO3, heating to above the melting point of LiNO3 (264° C.), cooling and then washing to remove the excess LiNO3 and side-reaction product    2. Treating the Na-ion material with an aqueous solution of lithium salts, for example 1M LiCl in water; and    3. Treating the Na-ion material with a non-aqueous solution of lithium salts, for example LiBr in one or more aliphatic alcohols such as hexanol, propanol etc.